(1) Field of the Invention
The present invention relates to a ceramic composite and a process for the preparation thereof. More particularly, the present invention relates to a ceramic composite composed mainly of hexagonal boron nitride and aluminum nitride, which is electrically insulating and has a high thermal conductivity, a relatively small thermal expansion coefficient, a low dielectric constant, and a good machinability. More specifically, the present invention relates to a ceramic composite which can be used as an IC package material, an IC substrate material, an electrically insulating radiating material with high thermal conductivity applicable at room temperature to a high temperature, and a machinable ceramic material.
(2) Description of the Related Art
Aluminum nitride has a high thermal conductivity of about 110 to about 195 W/m.K at room temperature, which is several times to ten times as high as the thermal conductivity of alumina. Accordingly, aluminum nitride has attracted attention as a substrate of a semiconductor device or a package material, to replace alumina, and is in practical use in this field (see, for example, Japanese Unexamined Patent Publication No. 61-270264, Japanese Unexamined Patent Publication No. 62-36069 and Japanese Unexamined Patent Publication No. 62-41766). This is because, to satisfy recent requirements for a reduction in the size of electronic devices, and to improve the performances thereof, an increase of the degree of integration, a multiplication of the functions, an increase of the operation speed, an increase of the output, and an enhancement of the reliability are being rapidly developed, with the result that the quantities of heat generated from semiconductors tend to increase correspondingly.
Nevertheless, several problems are encountered when aluminum nitride is utilized as an IC package material, an IC substrate material or an electrically insulating material. For example, although aluminum nitride has a higher thermal conductivity than alumina, and thus the problem of the heat radiating efficiency can be solved, the dielectric constant at room temperature and 1 MHz is as large as 9 to 10, and therefore, the delay of a signal in a wiring pattern formed thereon is prolonged, and accordingly, high-speed operation is hindered. Moreover, since the thermal expansion coefficient of aluminum nitride is larger than that of a single crystal of silicon, high-density packaging is difficult.
The machining of an aluminum nitride sintered body, as well as that of ordinary structural ceramics, is difficult, and the sintered body is defective in that a long time is required to machine it into a desired shape, and thus the machining cost is increased.
A hexagonal boron nitride sintered body has an excellent machinability, corrosion resistance, thermal shock resistance, and electrically insulating property, and therefore, this sintered body is used in many fields. In general, the hexagonal boron nitride ceramics is produced by the hot-press method using an oxide type assistant, and it is known that the sintered body obtained according to this method has a structural anisotropy attributable to the layered crystal structure of hexagonal boron nitride [see, for example, Bulletin of the Ceramic Society of Japan, Vol. 7, No. 4, page 243 (1972)]. This anisotropy is also found in the physical properties such as thermal conductivity, but even in a sintered body in which the anisotropy exists, the thermal conductivity is not very high, e.g., it has been reported that the thermal conductivity is only about 60 W/m.K at highest [see, for example, Electronic Ceramics, Vol. 17, No. 84, page 68 (1986)]. The reason for this is considered to be that a large amount of the oxide type sintering aids or a reaction product of the sintering aids with another component is present in the sintered body.
A sintered body composed solely of boron nitride has a relatively lower thermal conductivity than that of the aluminum nitride sintered body, but the thermal expansion coefficient of boron nitride is too low, and a good match with silicon, which semiconductor is to be actually mounted on a substrate or package, cannot be obtained. Accordingly, boron nitride is rarely utilized as an IC package material or an IC substrate material.
A sintered body formed by combining aluminum nitride with hexagonal boron nitride, mainly in an attempt to improve the machinability of the sintered body while maintaining the high thermal conductivity of the aluminum nitride sintered body, and a process for the preparation of this composite have been proposed. For example, Japanese Unexamined Patent Publication No. 60-195058 discloses a process for preparation of an aluminum nitride sintered body by adding at least one compound selected from the group of calcium nitrite, barium nitrite and strontium nitrite as a sintering aid in an amount of 0.01-5% by weight to an aluminum nitride powder or a mixed powder of aluminum nitride with boron nitride and/or silicon nitride, and Japanese Unexamined Patent Publication No. 60-195059 discloses a sintered body comprising aluminum nitride, boron nitride, and compounds of metals of the groups IIa and IIIa, in which polygonal aluminum nitride particles are filled in fracture planes and a thin layer of boron nitride is interposed in a part of, or all of the grain boundary.
It is stated that this sintered body is characterized in that it is a machinable ceramic composite which can be machined at a high speed by an ordinary tool. This sintered body is homogeneous and isotropic, and a value of 55 to 122 W/m.K has been reported as an example of the thermal conductivity thereof.
Nevertheless, although this sintered body has an excellent machinability, the thermal conductivity is 122 W/m.K at highest, and in some applications this thermal conductivity is not sufficient for an IC substrate material, an IC package material or an electrically insulating material.
Japanese Unexamined Patent Publication No. 58-32073 discloses a process in which an electrically insulating sintered body having a high thermal conductivity is prepared by incorporating up to 30% by weight of cubic or hexagonal boron nitride in a aluminum nitride, forming the mixture, and sintering the formed body in a vacuum or in a non-oxidizing atmosphere, or hot-press-sintering the formed body. This publication discloses that this sintered body has a thermal conductivity of about 85 to about 195 W/m.K, but does not disclose the anisotropy of the sintered body or the machinability of the sintered body with an ordinary tool or the like.
Furthermore, since these aluminum nitride/ boron nitride ceramic composites have a composition close to aluminum nitride, these ceramics are defective in that, like aluminum nitride, the dielectric constant is large and the thermal expansion coefficient is large.
Even in these aluminum nitride/boron nitride ceramic composites, in some certain preparation, as in the case of boron nitride which is sintered by hot-pressing, sometimes the structural anisotropy and the anisotropy of the physical properties attributed to the structural anisotropy of boron nitride are present (see Japanese Unexamined Patent Publication No. 62-56377), then the preparation process for eliminating this anisotropy and the way of the utilization of a material having a very small anisotropy has been focused and subjected for investigation.